The purpose of Project 2 is to examine whether aging is associated with an increase in the generation of ROS by skeletal muscle mitochondria that leads to damage by mitochondrial proteins and compromised mitochondrial function in muscle tissue, as predicted by the mitochondrial theory of aging, and whether exercise alters the effect. Skeletal muscle is a highly metabolic tissue and would therefore by particularly vulnerable to an accumulation of dysfunctional mitochondria, especially in the face of increased metabolic demand, such as during exercise. A positive feedback of oxidative damage would potentially lead to an accumulation of defective mitochondria, impaired energy production and a compromise in cellular function that would be amplified as the organism ages. Exercise would be predicted to increase the generation of ROS and exacerbate age-related alterations in mitochondrial dysfunction induced by ROS. The mitochondrial antioxidant enzyme Mn-superoxide dismutase (MnSOD) is the first line of defense against the primary source of ROS production, superoxide anions generated by electrons that escape the mitochondrial electron transport chain. In this study, I will use knockout (SOD2+/- and SOD2. 3-/-) and transgenic (SOD2T+/o) mouse models with altered MnSOD activity to test the mitochondrial theory of aging. The working hypothesis for this study is that aging is associated with increased ROS generation by muscle mitochondria, increased damage to mitochondrial proteins (in particular electron transport chain complexes) and compromised mitochondrial function. These processes will be exacerbated in the Sod2+/- and Sod2.3-/- mice; however, the damage and altered function will be prevented in Sod2Tg+/o transgenic mice with increased activity of MnSOD. The following specific hypotheses will be addressed: 1) Generation of ROS by mitochondria is increased with age and further amplified following isometric contraction. 2) Oxidative damage to mitochondrial proteins in resting hindlimb muscle will increase with age and isometric contraction will further increase the damage. 3) Mitochondrial function in skeletal muscle mitochondria is reduced with age and isometric contraction will further compromise function.